This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Protein-protein interaction is a process that controls the diverse biological functions. We will use SAXS to address the complex protein-protein interactions in solution by studying human proteases and bacterial toxins. First, we will study two human metalloproteases, insulin degrading enzyme (IDE) and presequence peptidase (PreP). IDE plays a key role in the clearance of insulin and amyloid beta, peptides vital for the progression of diabetes and Alzheimer[unreadable]s disease, respectively. PreP degrades mitochondrial presequences and amyloid beta so that it plays a key role to maintain integrity of mitochondria. We propose to use SAXS on study the oligomerization, the conformational switch, and substrate binding of these enzymes. Second set of proteins are bacterial toxins secreted by Bacillus anthracis, and Bordetella pertussis, the etiologic agent for anthrax and whooping cough. Our lab has solved the crystal structures of anthrax edema factor (EF), anthrolysin O (ALO), and pertussis CyaA. EF is an adenylyl cyclase toxin that binds anthrax protective antigen for its cell entrance and binds calmodulin for its catalytic activation. Pertussis CyaA also requires the binding of calmodulin for its catalytic activation. However CyaA has the calmodulin-binding site distinct from that of EF. For ALO, it needs to oligomerize for its pore-forming activity as well as binds to cell surface receptor. In addition, anthrax bacteria secrete approximately 400 proteins and of those, we have chosen several lipases and proteases to study their functions. SAXS will be used to gain the new insights for the interaction of these toxins with their partner proteins.